1. Field of the Invention
The present invention relates to a new and improved hip joint prosthesis.
2. Description of the Prior Art
Artificial hip joint prostheses have been long known to the art. At first, only replacement of the femur head was practiced. Now, operations for the replacement of the acetabular socket in the pelvis in addition to the femur head are commonly practiced.
Three major problems are commonly encountered in such operations. First is the problem of securely anchoring the prosthesis in the medullar bone cavity. Early attempts using inadequate anchorage often resulted in a resorption phenomena of the supporting bone. Such problems are explained in U.S. Pat. No. 3,894,297 to Mittelmeir et al. Such resorption phenomena caused the prosthesis shaft to loosen in the medullar cavity forcing the prosthesis further into the femur. Bone cement was later employed to circumvent this problem, but bone resorption and prosthesis loosening were still often encountered. Various other securing means have been used, such as screws (U.S. Pat. No. 3,781,917 and No. 3,782,373), screws and knife blades (U.S. Pat. No. 3,843,975), packed bone chips (U.S. Pat. No. 3,740,769), porous sintered powder (U.S. Pat. No. 3,808,606), teeth (U.S. Pat. No. 3,685,058 and No. 3,874,003), and ribs (U.S. Pat. No. 3,894,297). But anchoring the prosthesis in the medullar cavity remains a major problem.
Second, the strong resultant forces applied to the prosthesis during normal loading have often deformed or permanently damaged the artificial joint necessitating its replacement by a further operation. A variety of prosthesis configurations, as disclosed in the above patents, have been employed to overcome these resultant stresses with varying degrees of success.
Third, the problem of prosthesis wear remains a major concern. In many artificial joints, the ball head is mounted on a pin or arm connected to the femoral stem. Any binding or friction that develops at this connection may cause considerable attrition and may even require a further operation.
The present invention remedies all of these problems. First, the platform has a perpendicularly-extending lip on its lower lateral portion which defines a recess. This lip embeds into the cement during insertion of the prosthesis into the medullar cavity and the recess compresses the cement on the lateral side of the prosthesis thereby securely anchoring the prosthesis in the medullar cavity and resisting the later tendency of the prosthesis to rotate and tilt medially. A plurality of steps are also provided on the medial portion of the stem both anteriorly and posteriorly which compress the cement on the medial portion of the prosthesis and increase the contact surface thereby enhancing the anchoring effect. The T-shaped configuration of the stem also assists in anchoring the prosthesis in the medullar cavity.
Second, the orientation of the platform relative to the common axis of the ball head and the neck is such that the platform is more horizontal and thus more directly aligned with the resultant forces and can more evenly distribute these forces throughout the prosthesis and supporting bone and anchoring cement. These stresses are also dispersed in the present invention by the aforementioned steps along the stem and by the oval cross-sectional shape of the neck.
Third, the problem of wear is eliminated in the present invention because the prosthesis is one integral unit. No friction or resultant attrition is therefore encountered.